1. Field of the Invention
The present invention relates to an optical sheet, more particularly to an optical sheet by arraying several quantum dot fluorescent powders respectively on the reflective layer for increasing the light emitting efficiency, and the method manufacturing the same, light emitting diodes module and display using the same.
2. Description of the Prior Art
The typical lighting products normally use one or more incandescent lamps, although the incandescent lamp is cheap, but 90% of the energy is turned into the waste heat, only 10% of the energy is turned into the lighting. The efficiency is low and operation cost is quite high. In addition, the service life of the incandescent lam is very short.
As for the follow-up developed fluorescent lamp developing out in, although the efficiency is higher than the incandescent lamp, however, the material (such as mercury) is dangerous. In addition, the volume of fluorescent lamp is large and the cost is high, which is not suitable to be used in the small space, and the operation performance is poor under low temperature etc. Thus, although the service life of fluorescent lamp is longer than the incandescent lamp, the cost of the fluorescent lamp is higher than the incandescent lamp, and the fluorescent lamp needs the manpower for maintenance.
Thus, under the continuous advancement of technology, the Solid-State Lighting (SSL) has become an innovative lighting technology. It uses the light emitting diodes (LEDs), organic light emitting diode (OLED) or polymer (PLED) as the lighting source, in order to substitute the conventional incandescent lamp or fluorescent lamp.
Wherein, because the light emitting diode (LED) has the advantages of high illumination, small volume, light weight, uneasy to be damaged, low power consumption and long life etc., which is widely applied in the displaying products. Its light emitting principle is to utilize the combination of electrons and holes in the semiconductor, and the energy produced is released as light. The energy produced by the combination of electrons and holes is controlled by the energy level of material. When the energy level is larger, the wavelength of released light will be shorter. Thus, different material with different energy level can emit light with different wavelength. For example, the energy level of GaN is bout 3.39 eV. After the conversion, the wavelength is about 366 nm, which is the material for the emitting blue light. The appearance of the white light emitting diode extends the application of the light emitting diode to the illumination field. Upon comparing the white light emitting diode with the incandescent lamp and the fluorescent lamp, the light emitting diode has the advantages of low caloric value, low power consumption, long life, fast reaction and small volume etc., in which the power consumption is about ⅛ of the incandescent lamp, ½ of the fluorescent lamp, and the service life is more than 8,000 hours, which is 10 times of the fluorescent lamp.
At present, there are two kinds of ways to make the white light emitting diode mainly. One way is the single crystal light emitting diode way, namely utilizes the single light emitting diode crystal to match a variety of fluorescent powders to form white light. At present, the blue light emitting diode crystal and the yellow fluorescent powders are mainly utilized to obtain white light. The ultraviolet light emitting diode crystal is utilized to activate the blue, green and red fluorescent powders for mixing into white light. Another way is the multi-crystal light emitting diode way, namely encapsulates the red, green and blue crystals at the same time for mixing into white light. However, because the multi-crystal light emitting diode way needs to use several light emitting diode crystals, thus the cost is relatively high. In addition, because the driving voltage, luminous intensity, temperature characteristic and service life of every color crystal are different, so that not only the design and make are more difficult, but also after the use for a long time, the color coordinate is also easy to offset. Thus, the present development is relatively inclined to toward the single crystal light emitting diode.
In the methods used in the abovementioned light emitting diode, the simplest method is to use the blue light emitting diode crystal with yellow fluorescent powders for mixing into white light. Wherein, the Nichia Corporation used the yttrium aluminum garnet (YAG) to cover the blue GaN light emitting diode crystal firstly. Part of blue light emitted from the blue GaN light emitting diode is absorbed by the yttrium aluminum garnet to emit yellow light. The unabsorbed blue light is complemented with yellow light to form white light. However the preparation process of yttrium aluminum garnet in the present invention is much complicated, and the drawback is uneven light emitting often caused by uneven mixing.
In recent years, the development of quantum dots (QD) from novel light emitting material is very fast. The quantum dots are semiconductor nano-particles. The energy levels of the valence band and conduction band present discontinuity due to the influence of quantum confinement. The light emitting wavelength can be changed through adjusting the particle size of quantum dots. When the quantum dots are smaller, the light emitting color is approaching blue. When the quantum dots are larger, the light emitting color is approaching red. In addition, the quantum dots own excellent light absorbing-emitting characteristic, very narrow light emitting half-width, high efficiency and very wide absorption frequency. Thus, there is very high color purity and saturation degree. Through combining the abovementioned advantages, the quantum dots are considered as the potential white light emitting diode which can be used to replace current fluorescent powders.
However, the application of quantum dots on the technology developed for display has several drawbacks, such as uneven color and mutual color pollution etc.